One of the innovative technologies for atomizing viscous liquids and thin pasty materials in a gas is the use of shock waves. This method could be used in spray drying to disperse the liquid feed (see Chapter 14) and thus eliminate the conventional atomizer from the system. This will avoid problems with erosion in disk atomizers and clogging of single- or two-stream spray nozzles. Potential applications of this idea in other types vortex dryers, in which the momentum of the shock waves can enhance the hydrodynamic impact of a conventional gas carrier.
It should be noted that the term shock waves refers to a pressure wave of large amplitude that arises from sharp and violent disturbances when the velocity of wave propagation exceeds the velocity of sound propagation.
Characteristically, an abrupt change of the medium properties (e.g., pres-sure, stress, density, particle velocity, and temperature) takes place in a limited space across the shock wave (Schetz and Fuhs, 1996; Shapiro, 1953;
Anderson, 1982; Saad, 1993). In the case described in this chapter, the physical phenomenon of a shock wave is restricted to a one-dimensional plane wave propagation, in which the properties of air in the resonant tube of the wave generator vary with respect to only one spatial coordi-nate and time. Although it is technically feasible to generate a true shock wave by moving a piston into a compressible medium, the velocity of wave propagation in the system presented here is lower than the sonic exiting the generator is similar to a nearly discontinuous shock front tion for the pressure wave of such characteristics, the term shock wave is used throughout this chapter.
Atomization and drying in shock waves is still at the development stage, for example, laboratory- and pilot-scale experiments. Figure 10.1 presents schematically the experimental shock-wave generator used by Lyulin (1998) to disperse pasty materials in a spray dyer. The generator consists of a stainless steel tube 0.04 m in diameter, which propagates the pressure waves and serves as a quarter-wave (Helmholtz) resonator (see Chapter 14). One end of the resonance tube is open to the atmosphere, whereas the other end is connected to a pulse generator through a trun-cated cone 0.2 m long. The generator comprises a cylindrical chamber 0.08 m in internal diameter and a crankshaft-connecting rod assembly,
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propagating at a supersonic velocity. Because there is no adequate defini-of dryers for dispersed materials include pneumatic, ring, spin-flash, and
velocity. However, the almost flat pressure profile at the front of the wave
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144 Advanced Drying Technologies
which moves the piston in a reciprocating manner. The stroke of the pis-ton is 0.082 m. The pulse generator is driven by a 7 kW DC motor with controllable rotational speed, allowing variable frequency of the strokes from 7 to 28 Hz.
The instantaneous pressure and velocity distributions along the reso-nance tube are monitored by piezoelectric transducers and a hot-wire recorder. These make possible the tuning of the generator to the acoustic resonance by varying the rotational speed of the crankshaft. Also, there is a provision to attach resonance tubes of various lengths and diameters to obtain required amplitudes of pressure and velocity pulsations.
Figure 10.2 presents a typical temporal pressure oscillation at various lation near the piston is sinusoidal because of the reciprocating movement of the piston. As the pressure wave travels along the resonant tube, its descending part (the front of the wave) becomes shorter, whereas the steep pressure front characteristic of a shock wave. The extremely high gra-dients of pressure at the front of the plane wave not only initiate mechani-cal effects such as liquid dispersion but also can alter drying rates due to
cally unchanged along the resonant tube, and distortion of the sinusoidal velocity pulsation occurs only at the tube outlet. As shown in Figure 10.3, the amplitude of velocity pulsation decreases sharply with the distance of wave propagation in open air. To take full advantage of the momentum of the shock waves, liquid to be dispersed should be fed close to the outlet
Variable speed drive Oscillograph
Pulsator Resonator
Amplitude of velocity pulsation
Velocity
sensor Pressure sensor
rpm Meter
FIGURE 10.1
of drying instant pigments by the use of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Schematic of the experimental shock-wave generator. (From Lyulin, N. B., Intensification
anemometer, which are connected to the oscilloscope and photo film
points along the resonance tube. It is clear that the profile of pressure
oscil-ascending one (the tail of the wave) levels off gradually, finally giving a
Unlike pressure, the profile of the velocity pulsation remains practi-enhanced evaporation and capillary flow of the liquid and vapor moisture.
© 2009 by Taylor & Francis Group, LLC
from the resonant tube, in this case up to∼20 cm. Also, the strongest impact of shock wave due to air velocity might be expected over a distance of up to 1 m from the resonance tube.
Figure 10.4 presents amplitude–frequency characteristics of two
x = Lt (at piston)
x = 0.25 Lt
x = 0.5 Lt
x = 0 (at tube outlet) FIGURE 10.2
Characteristics of pressure oscillations in the resonance tube.
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different resonant tubes, where the first and the second resonance
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146 Advanced Drying Technologies
f=22.5 Hz
0 25 50 75 100 125
0 0.25 0.5 0.75 1.0 1.25 1.5
Distance (m)
Velocity (m/s)
f = 22.5 Hz
FIGURE 10.3
Variation of the amplitude of the pulsating velocity with distance from the outlet of the resonance tube.
8 10 12 14 16 18 20 22
6 0 50
100 Lt= 6.4 m
Lt= 5.8 m
Second resonance First resonance
Frequency (Hz)
Velocity (m/s)
FIGURE 10.4
Typical resonance characteristics of the shock-wave generator. (From Lyulin, N. B., Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Intensification of drying instant pigments by the use of shock wave generator, Ph.D. Thesis,
© 2009 by Taylor & Francis Group, LLC
respective curves.
For a given tube length, the resonance frequency is calculated as
f n v
r L
t
⫽ 2 ⫺1
( )
4 (10.1)where v is the velocity of sound propagation, Lt is the tube length, and n is the order of the harmonic frequency.
From Equation 10.1, it follows that the product fr ∙ Ltdoes not depend on the tube diameter but is a constant characteristic of the gaseous environ-nance is obtained at fr ∙ Lt= 82.5 m/s.
The maximum amplitude of the velocity pulsation appears at n = 1 (Figure 10.4). It is quite reasonable to assume that the amplitude of air pulsation at the tube outlet is proportional to the amplitude of the velocity of piston movement:
u⫽Kuc (10.2)
Accounting for the different diameters of the resonant tube (dt) and sub-stituting uc by (π(( fs), the following equation (valid for u <140 m/s) was
where s is the piston stroke length, dcis the piston diameter, and Z is a constant equal to 11.0 for 0.044 < dt< 0.05 m and equal to 9.1 for 0.033 <
dt ≤ 0.044 m. These values were determined experimentally.
Expressing the frequency in terms of the crankshaft revolutions per minute (rpm), the following experimental correlation can be used to cal-culate the length of the resonant tube and the amplitude of the air velocity at the tube outlet: momentum of the shock wave is able to disperse the liquid stream. By conditions can be construed as
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frequencies are clearly identified by the consecutive maxima on the
ment. Hence, at the sound velocity in air equal to 330 m/s, the first
reso-analogy with two-phase flows in packed columns, such hydrodynamic floodinggg. Hence, the region of stable at
omization is mapped in terms of a flooding coefficient, defined as the
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148 Advanced Drying Technologies
where A is a constant, dl is the diameter of the liquid jet subject to atomi-zation, and uland ug are the liquid jet velocity and the amplitude of the gas velocity pulsation, respectively.
where the liquid jet diameter, dl, is in millimeters; the frequency, f, is in hertz; and the gas velocity, ug, is in meters per second.
The dispersion of pasty pigments with shock waves was studied experi-mentally using a shock-wave generator with a resonance tube of 0.033 m.
This allows achieving air velocity equal to 163 m/s at 19.8 Hz of the recip-rocating movement of the piston. The pasty feed was located 0.05 m from the resonance tube outlet at its axis. Such a distance was found to be opti-mal over the range 1.5–5 times the tube diameter.
The following procedure was used to obtain a droplet size distribu-paper placed perpendicularly to the tube axis 1 m from the pasty feed.
under a microscope, and about 2000 droplets were counted for the size distribution. Knowing the dry mass of the dispersed pigment (m) and the total surface area of the spots (SS), the volume of the droplet having (X, % wet basis [w.b.]) moisture content that leaves the spot with diameter (dS) can be calculated from the following equation:
V d m
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shock-wave field:
ing coefficient:
tion (Muranov et al., 1997). Dispersed pigments were trapped in a filter Segments of the filter paper with spots of droplets were then analyzed mass flow ratio of undispersed liquid to the total liquid subject to the
Lyulin (1998) provided the following empirical correlation for the
flood-Thus, the generalized correlation for the flooding velocity shown in
© 2009 by Taylor & Francis Group, LLC
0 0.2 0.4
1.96 1.16 1.44 1.86 1.26 1.36 1.36 0.84
dl (mm)
1.78 2.51 7.28 7.28 7.28 7.28 7.28
0.42 1.05 1.15 0.73 0.94
7.8
of drying instant pigments by the use of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
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Flooding curve for atomizing liquids with shock waves. (From Lyulin, N. B., Intensification
© 2009 by Taylor & Francis Group, LLC
150 Advanced Drying Technologies
Thus, the droplet diameter is
d V d m
A typical distribution of droplet size for anticorrosive paint based on zinc phosphate is shown in Figure 10.6. The symbol dM
median diameter (midpoint of the size distribution), and dsVis the sur-face–volume diameter, that is, the diameter of the sphere having the same ratio of surface area to volume as the droplet and is calculated as
d d n
Figure 10.6 indicates that droplets generated by the shock wave are nearly monodispersed as the mass fraction of the droplets larger than Because the surface-volume diameter falls into the range of large drop-lets (106 µm), the median diameter appears to be more appropriate for evaluating the atomizing capability of shock waves. The experiments also proved that the size distribution depends on air velocity, and the median diameter decreases with decreasing velocity to reach the range 40.5–54 μm at 36.7 m/s (Lyulin, 1998). The best dispersion was found for air velocities >100 m/s.
The pilot plant experiments were carried out with various pigments (Table 10.1) dried in a spray dryer with the shock-wave atomizer designed according to the aforementioned procedure (Figure 10.7). Each test was run for 8 h to obtain a representative sample of ∼2000 kg of dry product.
dried under optimum conditions determined from laboratory tests, that is, at an inlet air temperature of 170°C and a feed rate of ∼120 kg/h. An average evaporation rate for all tests was ∼28 kg H2O/m3 h, which is mark-edly higher than for currently used spray dryers for which the volumetric evaporation capacity is ∼10 kg/m3 h.
Table 10.2 presents as an example the basic characteristics of zinc phos-phate dried in a pilot spray dryer with reference to industrial standards (Lyulin, 1998). It is evident that pigments dried in a spray dryer with the shock-wave atomizer satisfy all the requirements for the commercial product.
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whereas the total surface area of the dispersed liquid trapped in the filter
identifies here the
27 μm is negligible compared to the mass fraction of the fine droplets.
The filter cake at the initial moisture content from 50 to 60% w.b. was
© 2009 by Taylor & Francis Group, LLC
0 27 54 81 108 135 162 189 0
0.2 0.4 0.6 0.8 1.0
0 27 54 81 108 135 162 189
0 0.2 0.4 0.6 0.8 1.0
dM
dM
dsV
(a)
(b) dp (µm)
dp (µm)
FIGURE 10.6
Droplet size distribution for zinc phosphate–based anticorrosive paint at 163 m/s: (a) Frequency distribution (histogram) and (b) cumulative distribution. (From Lyulin, N. B., Mendeleyev University of Russia, Moscow, Russia [in Russian], 1998.)
In addition, pigments obtained with the shock-wave atomizer have a nar-rower size distribution, and therefore the throughput of the ball mills used in a downstream process of standardizing is increased by∼2 to 30%.
Although the results of this pioneering Russian study have docu-mented advantages of drying with shock waves, extension of this novel
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Intensification of drying instant pigments by the use of shock wave generator, Ph.D. Thesis,
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152 Advanced Drying Technologies
TABLE 10.1
Comparison of Selected Pigments Dried in a Spray Dryer with Shock-Wave Atomizer with a Reference Pigment
Pigment
Inlet Air Temperature
(°C)
Covering Power (g/m2) Grindabilitya(μm) Reference Product Reference Product Titanium dioxide
R-02 170 40 35 15 10
A-01 180 45 40 15 12
Lithopone 250 140 130 15 10
Chrome yellow
KL-1 200 60 60 15 10
K2H-1 200 50 45 15 10
OS 200 20 17 20 15
Zinc phosphate 170 — — 50 30
a After 30 min of grinding.
wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia (in Russian), 1998.
FIGURE 10.7
Schematic of the experimental setup for spray drying with shock waves.
Liquid feed
Pump Drying
chamber
Exhaust gas Resonator
Shock-wave generator
Heat exchanger
(cooler) Diffuser Confusor
Air
Product Heater
drying technology to other products will require additional studies, both theoretical and experimental. To our knowledge, Lyulin’s thesis
pendent tests.
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Source: From Lyulin, N. B., Intensification of drying instant pigments by the use of shock
is the only study of this novel concept. It is therefore difficult to make definitive recommendations about its potential without extensive
inde-© 2009 by Taylor & Francis Group, LLC
Appendix: Process Calculations and Equipment Design
The design criteria for a spray dryer with a shock-wave atomizer used by Lyulin (1998) are as follows:
Dryer type
• . Horizontal, concurrent, material fed to the zone of shock wave
Drying agent
• . Ambient air at 20°C and 70% relative humidity (RH)
Piston 0.08 m in diameter, stroke length 0.072 m
Motor 4 kW TABLE 10.2
Quality Indices for Zinc Phosphate for Anticorrosive Paint
Index Reference Pilot Dryer
Density (kg/m3) 3000 3000
Mass fraction of
Mass fraction of volatiles (%) <0.3 <0.5
pH of aqueous suspension 6.0–8.0 6.7
Oversize solids on sieve 0.063 mm (%)
<0.5 <0.05
Mass capacity (g/100 g pigment)
<50 <50
Whiteness, arbitrary units >92 >93
Degree of dispersion (µm) <30 <15
Note: White loose powder with no tendency to agglomerate.
of shock wave generator, Ph.D. Thesis, Mendeleyev University of Russia, Moscow, Russia (in Russian), 1998.
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Source: From Lyulin, N. B., Intensification of drying instant pigments by the use
Pasty filter cake (viscosity of ∼200 P) fed by the metering Initial material temperature 20°C, final material temperature
Initial moisture content >50% w.b., final moisture content
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154 Advanced Drying Technologies
Frequency 22.5 Hz
Spray angle 30°
Because the spray angle generated by the shock-wave atomizer is 30°, and the feeding point is at the outlet from the resonance tube, it is clear that the optimal drying chamber should be made as a truncated cone of at the outlet from the resonance tube located at the cone tip will decelerate while traveling along the cone axis until their terminal velocity reaches into a steady one, and the drying chamber can be shaped as a cylinder.
The following assumptions can be made for design calculations (Lyulin, 1998):
1. Droplets are spherical and do not collide during drying.
2. Droplet size and mass do not change during drying; heat- and mass-transfer area are equal to the total surface area of dry particles.
the shock wave in the plane of propagation.
5. The distribution of a dispersed material and a drying agent is uniform throughout the dryer volume.
All these assumptions except the third one are usual in engineering cal-culations for spray dryers. Assumption 3 might appear to be too general, but considering the novelty of shock-wave drying, it can be accepted as Experiments on the atomization of various pasty materials showed that 100 m/s is the minimum amplitude velocity required for the narrow droplet size distribution. Based on assumption 3, the maximum (i.e., initial) velocity of liquid droplets is thus equal to 100 m/s. Furthermore, the spatial varia-tion of droplet velocity will be the same as for the amplitude velocity of the
u⫽up⫽⫺14 11. eL⫹73 26. (10.12) Solution of Equation 10.12 for up
unsteady region of the dryer and hence determines the maximum length of the conical drying chamber. In a conventional spray dryer, however,
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included angle 30°. In such a configuration, the liquid droplets atomized
3. The velocity field of a droplet spray follows the velocity field of
6. The heat-transfer coefficient can be calculated using Nu= 2.
the first approximation to more accurate calculations.
shock wave shown in Figure 10.3 and is defined by the following equation:
= 0 gives L = 1.65 m, which identifies the the velocity of a drying air stream. At this point, the unsteady flow turns
4. The flow pattern in the dryer is a plug flow.
d
roplets evaporate in an air stream flowing at a superficial velocity from
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0.2 to 0.5 m/s. It is therefore reasonable to accept that the terminal velocity of droplets will be equal to the air velocity. Hence, for air velocity in a cylin-equal to 0.5 m/s, the real length of the conical drying chamber is 1.64 m.
Substituting the droplet velocity in Equation 10.12 for dL/dt, the follow-ing equation is obtained for the residence time distribution:
t dL
where the velocity of sound propagation in air at 20°C is 330 m/s.
Taking Z= 11, the maximum diameter of the resonance tube securing the amplitude velocity at least 100 m/s should be larger than
d Zd f s
tube and the drying air inlet should be concentric. Hence, the drying chamber is formed as a truncated cone with the inlet diameter (D) and the outlet diameter equal to the diameter of the cylindrical drying chamber (Dch). For an inlet diameter of 0.2 m, the chamber diameter is
Dch⫽D⫹2L tg ⫽ ⫹ ⭈ tg ⫽ m
velocity of the pasty feed can be calculated from Equation 10.7 as
u f u
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The length of the resonance tube for the first (maximum) resonance is
To obtain a uniform velocity profile in a drying chamber, the resonance drical part of the drying chamber (stationary region of the air-droplet flow)
Hence, the volumetric flow rate of the drying air is
Assuming the diameter of the material feeding tube to be 8 mm, the flow
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156 Advanced Drying Technologies
order of 1600–1700 kg/m3, the maximum capacity of the shock-wave atom-izer of these characteristics is in the order of 370–390 kg/h.
From the drying kinetics, it is clear that drying of pasty pigments that belong to the class of capillary-porous materials takes place mainly in the constant drying rate period. Hence, the drying time can be calculated from the heat-transfer equation as
t Q
h F Te
⫽ (10.14)
where F is the heat-transfer area, which is equal to the total surface area of
h k Nu
where the droplet diameter is taken from Figure 10.6.
The evaporation rate of water is
W G X X
Because the dryer throughput is limited by the capacity of the shock-wave
3
which is <390 kg/h, found as the maximum capacity of the shock-wave atomizer.
The density of drying air was calculated from the humid volume vH as
g
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Because the density of the pasty pigments from the filter press is in the
droplets, and h is the heat-transfer coefficient calculated from Nu = 2 as The maximum volumetric flow rate of the feed is
atomizer (airflow rate is equal to 0.458 m /s), the maximum feed rate is
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The humidity of the exhaust air equal to 0.088 kg/kg was taken as the maximum, yielding a dew point of the exhaust air of 50°C, which is the value needed to avoid condensation of water vapor in cyclones and bag Heat transferred to the dispersed material is used for sensible heating of
The humidity of the exhaust air equal to 0.088 kg/kg was taken as the maximum, yielding a dew point of the exhaust air of 50°C, which is the value needed to avoid condensation of water vapor in cyclones and bag Heat transferred to the dispersed material is used for sensible heating of